1. Field of the Invention
This invention relates to a temperature detecting method which makes use of a forward voltage of a diode, and more particularly to a temperature detecting method wherein an object of temperature detection element in the form of a semiconductor element and a temperature detecting diode formed in the proximity of the object of temperature detection element are thermally coupled to detect the temperature of the element by means of the temperature detecting diode.
2. Description of the Related Art
Conventional temperature detection which makes use of a diode is described with reference to FIGS. 1 to 6.
As a conventional example including a temperature detecting method, a block diagram of a semiconductor device of three terminal construction having a built-in overheat preventing function is shown in FIG. 1. Object of temperature detection element 102 is formed from a bipolar transistor, a field effect transistor or a like element and serves as a heat generating source. Heat 106 generated from object of temperature detection element 102 is transmitted to temperature detecting diode D1. Temperature detecting diode D1 placed at position p converts a temperature into a voltage in accordance with the relationship between temperature T(p) and forward voltage VF(p) of the diode illustrated in FIG. 2.
Overheat protecting circuit 103 obtains voltage VF(p) representative of the temperature of object of temperature detection element 102 from temperature detecting diode D1 and operates switch SW1 to switch on or off in response to the value of voltage VF(p) to control the supplying condition of the input voltage to object of temperature detection element 102 thereby to control the temperature of object element 102.
A construction of a pellet for realizing the block diagram of FIG. 1 is shown in FIGS. 3(a) and 3(b).
Object element 102 for temperature detection such as a bipolar transistor or a field effect transistor and temperature detecting diode D1 are formed in a neighboring relationship to each other on silicon substrate 101 by diffusion processing. Overheat protecting circuit 103 is formed at a location on silicon substrate 101 comparatively far from object of temperature detection element 102. Those elements are connected by aluminum wiring lines or the like so that they cooperatively form a semiconductor device having an overheat preventing function.
An example of temperature detecting method wherein object of temperature detection element 102 and temperature detecting diode D1 are formed in a neighboring relationship to each other on the same chip and thermally coupled to each other to detect the temperature of object element 102 for temperature detection by means of temperature detecting diode D1 in this manner is disclosed, for example, in Japanese Patent Laid-Open Application No. Heisei 01-196858. This document discloses that a temperature detecting circuit which makes use of a forward voltage of a diode is incorporated on a pellet on which a driving transistor is mounted, and the driving transistor and the temperature detecting diode are individually connected to a device. This example is described below.
FIG. 4 is a schematic view showing a structure of the essential part of object element 202 for temperature detection and temperature detecting diode D1 disclosed in the document. The example shown in FIG. 4 employs an N-channel field effect transistor of the enhancement type. In FIG. 4, a portion formed by terminals G, D and S is a field effect transistor serving as object element 202 for temperature detection, and another portion formed from terminals A and K serves as temperature detecting diode D1.
As described above, conventional temperature detecting diode D1 is formed such that it is included in a block adjacent object element 202 for temperature detection such as the block of overheat protecting circuit 203.
A concrete temperature detecting circuit employing temperature detecting diode D1 is shown in FIG. 5. Temperature detecting diode D1 is connected in a forward direction, and detecting current of several mA flows through temperature detecting diode D1. Voltage VF(p) across temperature detecting diode D1 is compared with reference voltage Vref set by resistors R7 and R8 by comparator Cmp1, and voltage Vo is outputted from comparator Cmp1. An example of temperature detecting circuit of the type described above is disclosed in Japanese Patent Laid-Open Application No. Showa 56-120153 or Japanese Patent Laid-Open Application No. Heisei 1-114060.
The temperature detecting circuit disclosed in Japanese Patent Laid-Open Application No. Heisei 56-120153 is constructed such that a diode for detecting the temperature is disposed adjacent another pellet or the same pellet in the proximity of an object element for temperature detection such as a transistor or an IC, and a temperature detecting circuit which makes use of the forward voltage of the diode is incorporated in the outside or the inside of the IC and the forward voltage of the diode including the temperature information is compared with a reference voltage by a comparator or an operational amplifier to control the current flowing through the IC to protect the device. Also overheat protecting circuit 103 shown in FIG. 1 has a similar construction to that disclosed in Japanese Patent Laid-Open Application No. Showa 56-120153.
The temperature detecting circuit disclosed in Japanese Patent Laid-Open Application No. Heisei 1-114060 is constructed such that a temperature detecting circuit including a transistor for detecting the temperature is disposed in the proximity of an object element for temperature detection and makes use of a forward voltage of the diode between the base and the emitter of a transistor forming an IC to control the current of a power source connected to the IC to protect the device.
The temperature distribution on the section taken along line Z-Z' of the schematic views of FIGS. 3(a) and 3(b) including temperature detecting diode D1 immediately after a high power is instantaneously applied to the semiconductor device shown in FIGS. 3(a) and 3(b) is such as that shown in FIG. 6.
The temperature distribution on the section taken along line Z-Z' usually exhibits the highest temperature (Tmax) at the location of object element 102 for temperature detection, and the temperature of temperature detecting diode D1 spaced by distance p from object of temperature detection element 102 is T(p) which is lower than temperature Tmax of element 102.
Where the applied power is represented by P, the maximum temperature of element 102 is represented by Tmax and the thermal resistance between temperature detecting diode D1 at position p and element 102 is represented by Rth(p), temperature T(p) at position p of temperature detecting diode D1 in the conventional example is given by following equation 1: EQU T(p)=Tmax-P.times.Rth(p) equation 1
Further, when sufficient time elapses after power is applied to the semiconductor device, the temperatures of both element 102 and temperature detecting diode D1 approach a substantially uniform temperature.
However, in the prior art described above, as seen from FIGS. 3(a) and 3(b), a thermal resistance originating from the thermal conductivity and/or the distance between object of temperature detection element 102, which also serves as a heat generating source, and temperature detecting diode D1, and particularly when a high power is instantaneously applied or in a like case, a difference arises between temperature Tmax of element 102 and temperature T(p) of temperature detecting diode D1 as seen in FIG. 6. Consequently, where the prior art is applied to an overheat protecting circuit or a like circuit, there is a problem in that the temperature of element 102 may become higher than an overheat protecting temperature set in advance by resistance R7 and resistance R8 of the temperature detecting circuit of FIG. 5.
This problem is not significant while the input power to the semiconductor is low. However, when a high power is inputted instantaneously, the problem becomes significant. Accordingly, not only realization of a semiconductor device having an overheat protecting function or a like function is difficult, but also there is the possibility that the life of the semiconductor device may be reduced by inadvertent application of a high power.